The hard copying for image duplication generally relates to the screening and plate-making technology for printers and the advanced printing and plate-making device. The screening technology used for duplicating an image with hard copying is also called the digital image halftone technology. The digital image halftone technology comprises amplitude-modulation (AM) screening and frequency-modulation (FM) screening. The amplitude-modulation screening technique is also called ordered dithering of gathered dots, characterizing in that, the colored points in a produced halftone image are gathered in pairs geometrically to form clusters of colored regions called dots. Since the technology controls the size of the dots to represent the gray level of the original image, the dots are called amplitude-modulation dots.
For the mixed screening technology based on the FM and AM screens, the applicants have filed a Chinese Patent Application “method for frequency-modulation screening using error diffusion based on dual-feedback” (Application No. 200510068127.8, published on Sep. 14, 2005), wherein the disclosed technology of FM-AM mixed dots is mainly based on the apparatus with single-bit imaging depth. During the practical output, since the single-bit apparatus has only two gradations (white: 255; black: 0) for outputting dots, the phenomenon of sawtooth exists generally in the margins of the FM-AM mixed dots output by the single-bit depth imaging apparatus, especially in the margins of characters. Furthermore, because of the AM feature of the mixed dots, the aggravation of image sharpening due to the size of the dots results in severe granular sensation and texture in the image output by the single-bit depth imaging apparatus so as to impact the smoothness in the whole image and the quality of the output image.
To overcome the shortcomings of the single-bit depth imaging apparatus, the multi-bit depth imaging apparatus appears. The output gradations of the apparatus are increased up to the power of 2 depending on the imaging depth. The general 2-bit or 4-bit output apparatus can represent 4 or 16 gradations respectively. The final object of the multi-bit depth imaging apparatus is to obtain the output effect with high resolution under low resolution so as to overcome the problem of imaging quality under low resolution. Furthermore, in combination with the PWM gradation offset technology embedded in the multi-bit depth imaging apparatus, the halftone dots are output stably and the optimized quality of the halftone dots output by the apparatus is obtained.
In the field of hard copying, the halftone screening technology adapted to the multi-bit depth imaging apparatus is mainly the AM screening technology. Since the AM dots have inherent regular distribution and the size of the dots has controllability, it is easy for the conventional AM screening technology with single-bit depth to evolve corresponding screening technology with multi-bit depth adapted to the multi-bit depth imaging apparatus.
In the application “method for frequency-modulation screening using error diffusion based on dual-feedback” based on the error diffusion, it is hard to deduce the dot generation technology with multi-bit depth due to the real-time dynamic halftone screening, the randomicity of distribution of the dots and the random change of controlling the shape of the dots in different positions. No effective methods for generation are existing and the conventional methods are needed to be improved.